mitsingen
to sing along
To sing together with other people or along with a melody that you hear.
Als die Band ihren größten Hit spielte, hat das ganze Stadion mitgesungen.
When the band played their biggest hit, the whole stadium sang along.

to sing along
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to sing along
To sing together with other people or along with a melody that you hear.
Als die Band ihren größten Hit spielte, hat das ganze Stadion mitgesungen.
When the band played their biggest hit, the whole stadium sang along.

'Mitsingen' (to sing along) is a separable verb. In a main clause, the prefix 'mit' goes at the end. In a subordinate clause and in the infinitive with 'zu', the verb stays together.
Hörst du die Musik? Sing doch mit!
Do you hear the music? Sing along!
The prefix 'mit-' expresses that you do something together with others. So 'mitsingen' (to sing along) literally means 'to sing together with others.'
Die Kinder sangen im Chor begeistert mit.
The children enthusiastically sang along in the choir.
The verb can be used without an object when the general act of singing along is meant. It is often used with the preposition 'bei' to say which part someone sings along with.
Beim Refrain haben alle laut mitgesungen.
Everyone sang along loudly during the chorus.
to sing
'singen' and 'mitsingen' both describe making melodies or songs with your voice. The difference lies in the situation: you can 'singen' alone or in front of others, without anyone else singing. 'mitsingen' is used when a song is already playing or other people are singing and you join in. The words can be confused because both involve singing. As a practical rule: if someone is performing the song themselves, you say 'singen'. If someone joins in with a song or with a group, you say 'mitsingen'.

Beim Konzert kannte Lisa den Refrain und hat laut mitgesungen.
At the concert, Lisa knew the chorus and sang along loudly.
Im Musikunterricht singt die Klasse ein neues Lied.
In music class, the class sings a new song.